As the movie — a combination of Her and Him versions that focus on the wife's and husband's perspectives — explores each character's psyche and how the couple have been affected by the tragedy and separation, there's a lot of their past that remains unexplained and unseen. Viewers slowly learn that Eleanor and Conor lost their child shortly before she disappeared, but exactly what happened to the kid is never explained. And the audience only gets a few glimpses of what Eleanor and Conor were like when they were together.

But both McAvoy and writer-director Ned Benson told The Hollywood Reporter that the details of what happened are unnecessary.

"Would the manner in which the child dies make their grief any more? Any less? Would it make their grief more interesting to an audience? Less interesting to an audience? So it just felt like to go into that would have been needlessly…crude or something," McAvoy said at Wednesday's Cinema Society- and Prada-hosted screening in New York.

The actor added that he thinks it's more important to the film to see how the characters are dealing with that tragedy.

"I think Ned was trying to make a film about what you do after the event. How you put yourself back together. Not getting into the ins-and-outs of what happened to cause the breakup," McAvoy said. "The grief that [Eleanor and Conor] carry has to be fully realized by the actors, and we know exactly what happened and we agreed on that and we worked on that and we know what that is. But, yeah, I think as soon as you start to deal with what happened in the past, immediately you're not as interested as what happens in the future."

Benson also said that he intentionally left some details ambiguous.

"I just wanted to more insinuate things and…let people ask questions and make assumptions about these [characters]," he told THR. "This was a matter of a relationship. This is about people going through something in a certain time period…I wanted the behavior to sort of dictate who these people were rather than like telling you exactly what happened. It was more important for me to sort of let them unfold as characters and let their story unfold through their work."

But McAvoy said he and Chastain did fill in all of the missing details about their characters and Conor's and Eleanor's past.

"The first thing that Jessica and I did was agree upon how we met and all the places we'd been around the world together and all that and where we visited. What we liked to do. What made us tick," he explained. "How we agreed on trying to have a baby. Whether the baby came as an accident or as a planned pregnancy…. And then we started to talk about the exact timeline of what happened to the baby: how we went to the hospital…."

Chastain added that she went so far as to figure out parts of her character that maybe even Benson didn't know.

"Whenever I'm playing a character, it's important for me to always have secrets, some things that maybe even the director doesn't know," she told THR. "I created this imagination of this history for Eleanor because then I can be in the movie and someone mentions something from Eleanor's past, it'll click in."

And TWC is set to release the Her and Him versions in theaters Oct. 10, for those who want to take a deeper dive into the female and male sides of the story.

Benson explained that those versions include several scenes that didn't make it into the combined movie and more of the supporting characters, played by Viola Davis, William Hurt, Bill Hader, Jess Weixler and others, since their stories exist within those of Eleanor's and Conor's.

With those versions, Benson also has said he wanted to explore the ideas of memory and subjectivity, and Chastain noted that with those films, you can see how different viewpoints color the truth.

"They can see the idea of perspective and how — perhaps they look back to their past breakups or even the beginning of a romance — your idea of what happens may not be the other person's idea of what happened. And I find that very interesting, the idea of the truth," she said.